Motor-driven power steering unit

ABSTRACT

A motor-driven power steering unit comprising an electric motor for producing an auxiliary steering force in a steering system coupling a steering wheel to two wheels of a vehicle. The steering system provides assistance, based on steering information, to a rotation of the steering wheel by control of an electric current fed to the electric motor. A steering angle detection device is provided, wherein the steering system regulates the assistance as a function of a steering angle detected by the detection device. The assistance, or current, is reduced after a pre-set or pre-definable angle is reached or exceeded prior to reaching a respective end position. The reduction of the assistance or current is cancelled as soon as signs of steering force and the steering angel no longer coincide.

BACKGROUND OF THE INVENTION

The present invention relates to a motor-driven power steering unit withan electric motor for producing an auxiliary steering force in asteering system that couples a steering handle to the wheels of avehicle, whereby the power steering unit assists the rotation of thesteering wheel by controlling an electric current fed to the electricmotor based on steering information comprising a steering angledetection device.

A generic motor-driven power steering unit is known from DE 38 21 789.This power steering unit comprises a steering force protection devicefor detecting the steering force produced in the steering system as wellas an overload protection device to reduce the electric current fed tothe electric motor. On the basis of the steering force detected, whichis measured, for example, on the torsion bar of the steering system, thesupport is reduced by means of the overload protection device if thesteering force detected exceeds a pre-determined angle. In addition, DE38 21 789 discloses a generic power steering unit, in which additionallythe steering speed is measured and this variable is also considered inthe computation or reduction of the support. With this embodiment, thesupport is only reduced if, on the one hand, the hand force level isexceeded and, on the other hand, the steering speed has not exceeded acertain value. The power steering units known from DE 38 21 789 have thedisadvantage that the driving feel at the end stop is determined by therotational rigidity of the torsion bar of the steering system and cannotbe influenced or regulated by controlling the power steering unit.

Object of the present invention is to create a power steering unit, inwhich the driving feel within range of the left and right end stop canbe influenced in a pre-determinable way i.e. by a correspondingreduction of the support.

SUMMARY OF THE INVENTION

This object is achieved according to the invention in each case by meansof a motor-driven power steering unit that comprises an electric motorfor producing an auxiliary steering force in a steering system thatcouples a steering wheel two wheels of a vehicle, wherein the powersteering unit provides assistance, based on steering information, to arotation of the steering wheel by a control of an electric current thatis fed to the electric motor, and also comprises a steering angledetection device, and also comprises a steering angle detection device,wherein the power steering unit regulates the assistance as a functionof a detected steering angle, wherein the assistance, or current, isreduced after a preset or pre-definable angle is reached or exceededprior to reaching a respective end position, and wherein such reductionof the assistance or current is cancelled as soon as the signs ofsteering force and steering angle no longer coincide.

The power steering unit according to the invention is advantageouslycharacterized in that use of steering angle information alreadyavailable in any modern steering system can be made. Advantageously, thesupport or the current to be supplied, which has been detectedbeforehand by the motor control, can already be reduced before therespective end stop is reached. The extent of the reduction in this caseadvantageously depends on the steering angle, so that the support can bereduced more and more the nearer the respective end stop is reached.Advantageously, the reduction can result, for example, due to the factthat the value detected by the motor control for the current to besupplied is multiplied by a proportionality factor a and then the valueis transmitted to the controllable power source, which then regulatesthe current to be supplied. The function a=f (steering angle) in thiscase can be selected at will. If this support is to be prevented frombeing totally reduced, that is to say to zero, it is advantageous tospecify a lower minimum value for the proportionality factor.

It is also advantageous that the reduction of the support only becomesactive if the vehicle has not yet exceeded a certain speed. Thereby,erroneous detection of the steering angle only having an effect at lowspeeds is prevented and no disturbance arises at higher speeds throughinadvertent influence of the support.

Likewise the range, within which the support is to be reduced, can befreely specified by pre-setting limit angles independently of technicalparameters.

So that the steered vehicle wheels can be turned with the full supportout of one of the two end positions to the central position, the powersteering units according to the invention possess a function, whichquery the signs of the hand force and the steering angle. In this case,depending on the sign convention for each end position two cases aredifferentiated. If both variables have the same sign, which means thatthe steering force is directed in such a manner that attempt is made tomove the wheels toward the end position, the support is reduced, if theaforementioned limit angle is exceeded. When the wheels are steered outof the end position toward the central position on the other hand, thesigns of the variables are different. In this case, the maximum oroptimum support is to be made available to the driver. Theproportionality factor in this case therefore is set to one, so that thevalue computed by the motor control for the current to be supplied ispassed onto the variable power source.

Naturally, the invention is not limited to the fact that the value ofthe current to be supplied is only multiplied and handed over to themotor control or the controllable power source by means of theproportionality factor. It is also possible that the support forcecomputed by the steering system is multiplied with the proportionalityfactor and then the product is handed over to the motor control and/oranother control or regulating unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The operational principle of the invention is described in detail belowon the basis of drawings, wherein:

FIG. 1 shows angle ranges, in which the support is reduced, if thevehicle speed has not exceeded a certain value;

FIG. 2 shows a possible function a=f (steering angle);

FIG. 3 is a flow chart for a possible process to compute aproportionality factor;

FIG. 4 is a block diagram for a possible arrangement of the steeringangle dependent-reduction unit or “endlock protection” unit in thesteering system.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 serves to explain the variables relevant to the power steeringunit according to the invention. In this case, “endlock_right”designates the angle, at which the steering system is positioned in theright mechanical end stop. “endlock_left” designates the angle, at whichthe steering system is positioned in the left mechanical end stop. Theangle Φ_(gr) is the right limit angle and Φ_(gl) is the left limitangle. If the steered wheels are rotated in such a manner that thesteering angle falls in the hatched area, the support or the current tobe supplied is reduced. If optionally the vehicle speed v_(vehicle) isconsidered, the support or the current to be supplied is only reduced,if the vehicle has not exceeded a certain preset speed. Depending on thesign convention selected, the steering angle Φ_(sw) is positive, if thesteering wheel or the wheels are moved to the right. The steering angleΦ_(sw) on the other hand is negative, if the steering wheel or thewheels are turned or moves to the left. If the steering wheel and/or thesteered vehicle wheels are in the central position, the steering angleΦ_(sw) is equal to zero.

FIG. 2 shows a possible functional correlation between the factor a andthe steering angle Φ_(sw). The factor a is equal to one, if the steeringangle Φ_(sw) has not exceeded either of the two limit angles Φ_(gr) andΦ_(gl). On the other hand, as soon as the steering angle Φ_(sw) hasexceeded one of the two limit angles Φ_(gr) and Φ_(gl), the factor abecomes less. If necessary, additional query and limitation for thefactor a can result. Thus, for example, the value can be limited to alower limit value min_support, which it reaches with the steering angleΦ_(sw) equal to endlock_right or endlock_left or beforehand. Thefunctional correlation shown in FIG. 2 is only given by way of example.Naturally, it is possible to preset the curve progression differentlydepending on need. For example, a linear, quadratic, exponential orlogarithmic decrease of the factor a in the angle rangesendlock_left<Φ_(sw)<Φ_(gl) or Φ_(gr)<Φ_(sw)<endlock_right isconceivable. Thus, any appropriate values for min_support can also bespecified.

FIG. 3 shows a flow chart for a possible process to implement the powersteering units according to the invention. In a first step 100, it ischecked whether the vehicle speed has exceeded a certain valuev_(limit). If the vehicle is travelling at a speed greater thanv_(limit), the proportionality factor support_factor is set in step 1000equal to one, whereby the computation process is continued to the end,and is afterwards restarted. If the vehicle has not exceeded v_(limit)it goes to step 200, where the sign of the steering angle Φ_(sw) ischecked. If Φ_(sw) is less than zero, that is to say turned to the left,the factor a is computed by means of the step 400. The angle differencebetween the end stop angle endlock_left and the steering angle Φ_(sw) iscomputed in this case. This difference is divided by the angledifference of endlock_left minus Φ_(gl). Since the angle differenceendlock_left minus the limit angle Φ_(gl) is a constant, which indicatesan angle range, it can also be divided by this previously computed andstored angle range value. If the last mentioned angle difference ispositive, the absolute value of a does not need to be computed.Accordingly, the factor a is computed in step 300, if the steering angleΦ_(sw) is greater than zero. The factor a is a function of the steeringangle Φ_(sw). Depending on the preset limit angle Φ_(gl) or Φ_(gr), thevalue of the factor a, in the case of a steering angle Φ_(sw) becominglesser or greater, will fall short of a certain pre-definable value. Instep 500, it is checked whether the value previously computed in steps300 and 400 is greater than one. If, in the case of the process shown inFIG. 3, the value of a is greater than one, the proportionality factorsupport_factor is set equal to one and the process is ended or the valueof the proportionality factor support_factor is passed on for furthercomputation of the support. If the value of the factor a is less orequal to one, the signs are checked in step 600. In this case,alternatively either the direction or the sign of the hand forcetq_(sensor) can be compared with the sign of the steering angle. Or thedirection or the sign of the support or the controlling signal tq_(reg)already detected by the steering system or controller is compared withthe sign of the steering angle. If the signs or directions do notcoincide, it goes to step 1000 and the proportionality factorsupport_factor is set equal to one. If the signs of the values areunequal, the case results that attempt is made to move the wheels or thesteering wheel towards the central position. If the signs are identical,attempt is made to move the wheels or the steering wheel towards the endposition. In this case, it is checked in step 700 whether the value ofthe factor a is less than the minimum permissible support factormin_support. If a<min_support, it goes to step 800 and theproportionality factor support_factor is set to equal min_support. Ifa>=min_support, it goes to step 900 and the proportionality factorsupport_factor is set to equal a. After steps 800 and 900 are completed,the computation process is concluded and the computation of a newproportionality factor support_factor can be started.

FIG. 4 shows a block diagram for a possible arrangement of the steeringangle dependent-reduction unit or “endlock protection” unit in thesteering system. The controller produces a controlling signal tq_(reg),which is multiplied with the proportionality factor support_factor. Theproduct of the multiplication is then fed to the motor control of theelectric motor, which generates the support. The proportionality factorsupport_factor is computed by means of the “endlock protection” unit,which for example uses the process described in FIG. 3. Only absolutelyessential input variable for the “endlock protection” unit is thesteering angle Φ_(sw). Optionally, the vehicle speed v_(vehicle) and thehand force detected or the size of the adjusting signal tq_(reg) canalso be used to determine the proportionality factor support_factor.

The specification incorporates by reference the disclosure of Germanpriority document 101 17 634.1 filed Apr. 9, 2001 and PCT/EP02/03884filed Apr. 8, 2002.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

1. A motor-driven power steering unit, comprising: an electric motor forproducing an auxiliary steering force in a steering system that couplesa steering wheel to two wheels of a vehicle, wherein said steeringsystem provides assistance, based on steering information, to a rotationof said steering wheel by a control of an electric current that is fedto said electric motor; and a steering angle detection device, whereinsaid steering system regulates said assistance as a function of asteering angle detected by said steering angle detection device, whereinsaid assistance, or current, is reduced after a preset or pre-definableangle is reached or exceeded prior to reaching a respective endposition, and wherein the reduction of said assistance or current iscancelled as soon as signs of steering force and the steering angle nolonger coincide.
 2. A motor-driven power steering unit according toclaim 1, wherein said power steering unit has an overload protect ondevice to reduce the electric current fed to the electric motor.
 3. Amotor-driven power steering unit according to claim 1, wherein saidassistance or electric current is reduced in angle ranges prior toreaching respective end positions of steered vehicle wheels.
 4. Amotor-driven power steering unit according to claim 1, wherein saidassistance is reduced in a manner that steered wheels or said steeringwheel or steering linkage can at maximum only reach the end positionswith a pre-set maximum speed or force, or with said pre-set max mumspeed and said force.
 5. A motor-driven power steering unit according toclaim 1, wherein said assistance is reduced only if the vehicle has notexceeded a certain speed.
 6. A motor-driven power steering unitaccording to claim 1, wherein said assistance is continually reducedmore and more after a certain pre-definable or pre-set angle Φ_(gr) orΦ_(gl) is reached until a respective end position is reached.
 7. Amotor-driven power steering unit according to claim 6, wherein saidreduction increases linearly, exponentially, quadratically orcorresponding to an angle dependent-function.
 8. A motor-driven powersteering unit according to claim 1, wherein said assistance computed bysaid steering system, of the value of the electric current to besupplied, is multiplied with a proportionality factor “support₁₃factor”, and the resulting product is fed to said electric motor.
 9. Amotor-driven power steering unit according to claim 8, wherein the“support_factor” is computed by the steps of: checking in a firstprocess step whether an actual vehicle speed v_(vehicle) has exceeded acertain speed v_(limit), and if v_(vehicle)≧v_(limit), setting theproportionality factor “support_factor” to equal 1 (one), and theprocess ends, or if v_(vehicle)<v_(limit), continuing the computation ofthe proportionality factor “support_(—factor”.)
 10. A motor-driven powersteering unit according to claim 9, wherein if the conditionv_(vehicle)≧v_(limit) is present, checking in a subsequent process stepwhether the steering angle is positive or negative, whereby a factor ais then computed subsequently for the respective angle ranges, whereby$a = {\frac{\varphi_{s\; w} - {endlock\_ left}}{{endlock\_ left} - \varphi_{g\; l}}}$if said steering wheel is turned from a central position to the left,and whereby$a = {\frac{{endlock\_ right} - \varphi_{s\; w}}{{endlock\_ right} - \varphi_{gl}}}$ if said steering wheel is turned from a central position to the right,whereby endlock_right is the right stop angle, endlock_left is the leftstop angle, and Φ_(sw) is the actual steering wheel angle.
 11. Amotor-driven power steering unit according to claim 10, wherein forvalues of the factor a which are greater than the value one, theproportionality factor support_factor is set equal to one and thecomputation is ended.
 12. A motor-driven power steering unit accordingto claim 10, wherein the proportionality factor support_factor is setequal to the factor a if the value of the factor a is greater than orequal to a pre-set minimum value min_support and less than or equal toone, and wherein the proportionality factor support_factor is set equalto the minimum value min_support if the factor a is less than theminimum value min_support.
 13. A motor-driven power steering unitaccording to claim 10, wherein the proportionality factor support_factoris set equal to one as soon as the signs of the steering force and thesteering angle no longer coincide.